Architects of high-performance computers and other electronic devices have long been forced to acknowledge the existence of a large gap between the speed of the central processing unit (CPU) and the speed of attached mass storage devices such as magnetic hard disk drives.
In contrast to primary memory technologies, the performance of reading data from conventional magnetic disk drives and other such mass storage devices has improved only modestly. In these mechanical devices, the delays in retrieving stored data are dominated primarily by seek and rotation delays.
For devices such as disk drives, data is stored on a data storage platter having a plurality of concentric tracks. The data on each track is further divided into arcuate sectors.
A read/write head is used to read the data. The data is read by moving the head over a particular track and scanning the sector(s) containing the data.
In such read operations, it is often necessary to move the head to new locations in search of sectors containing specific data items. Such relocations are usually time-consuming. The access time (.tau..sub.a) is defined as the average time spent in going from one randomly selected spot on the disk to another. .tau..sub.a can be considered the sum of a seek time (.tau..sub.s) , which is the average time needed to acquire the target track, and a latency (.tau..sub.a), which is the average time spent on the target track waiting for the desire sector. Thus, EQU .tau..sub.a =.tau..sub.s +.tau..sub.l
Currently, in prior art devices, the latency is half the resolution period of the disk since a randomly selected sector is, on the average, halfway along the track from the point where the head initially lands. Thus, for a disk rotating at 1200 rpm .tau..sub.l =25 ms, while at 3600 rpm .tau..sub.l .apprxeq.8.3 ms. The seek time, on the other hand, is independent of the rotation speed, but is determined by the traveling distance of the head during an average seek, as well as by the mechanism of head actuation.
Due to cost constraints and reliability concerns, it is not feasible to construct a disk having a speed greater than 7,200 rpm and, in fact, most drives today operate at 3,600 rpm. However, as stated above, there is a need to increase disk drive performance in order to satisfy the demands of today's higher performance computers.
The present invention satisfies the above-discussed problems using two or more heads for reading data that is redundantly stored in various locations offset from each other.